Watercraft for transporting cargo have long been built of single external hulls. The large size and load of tankers, such as those designed for the transport of crude oil or other fluid cargo, entail special structural requirements. A long history of experience with single hull tankers, however, has led to established design rules and guidelines for structural safety. Engineering principles have gradually been adapted from smaller ships to ever larger ones over a long span of time.
Experience with double hull tanker designs, on the other hand, is much more limited. Nevertheless, Congress has enforced the abrupt adoption of double hull designs by passing the Oil Pollution Act of 1990, or "OPA 90," following the environmental disaster caused by the Exxon Valdez oil spill. The outer hull of a double hull tanker serves as a sacrificial barrier, to absorb any impact in accidents, such as collisions or grounding. The outer hull thus minimizes the risk of puncturing the inner hull and consequent spills like that of the Exxon Valdez. As single hull tankers are gradually retired from service to comply with OPA 90, they are being replaced by double hull designs. Those designs lack the benefit of time testing to establish adequate margins of safety.
FIG. 4 depicts the cross-section of a conventional double hull tanker 40. The longitudinal dimension of the tanker 40 in this view would extend into and out of the paper. Typically, as is known in the art, an outer hull 42 of the tanker 40 is formed by metal plating reinforced by longitudinal stiffeners in the form of welded T-beams, L-beams or extruded bulb-beams. The plates are modularly connected to web frames which extend laterally across the width of the tanker 40. The stiffeners of the plates are fitted through slots on the web frames, and the joints are welded. An inner hull 44 may be similarly formed.
Recent inspections of older double hull tankers have revealed major structural failures at or near the double bottom area. In particular, longitudinal cracks have been found in bottom plating 54 of the inner hull 44. These fractures were centered between lateral frames and extended as long as three quarters of the distance between frames, extending along the fillet welds which fix the longitudinal stiffeners to the plating, on either side of the stiffeners. Additional fractures were found at the junction between the longitudinal stiffeners and the web frames in both the inner bottom 54 and the outer bottom 50.
Accordingly, a need exists for large watercraft which avoid such structural failures while providing the advantages of a double hull construction.